The recent literature concerning the commercial use of chlorine has focused on the small but measurable quantities of polychlorinated aromatic compounds such as chlorobenzenes, polychlorinated biphenyls (PCBs), polychlorinated dibenzodioxins (PCDDs) and polychlorinated dibenzofurans (PCDFs) all of which are hereinafter referred to as PCDD/F(s) and are considered environmentally toxic. These highly undesirable compounds (HUCs) are found in soil and lake sediments and their reduction or elimination from industrial production is becoming of paramount concern. The literature contains many references to PCDD/F formation which in general support the position that the formation of trace quantities of PCDD/F's is unavoidable in many industrial chlorine containing processes involving addition or removal of chlorine atoms from non-aromatic hydrocarbons. This is particularly true for oxychlorination processes, and especially true for the oxychlorination of ethylene to 1,2-dichloroethane, also referred to as ethylene dichloride (EDC), an important intermediate in the production of vinyl chloride monomer (VCM).
Because of the concerns about the toxicity and bioaccumulation of polychlorinated aromatics, minute quantities are being monitored in a variety of industries, including solid waste incinerators, EDC/VCM processes and other processes that involve the chlorination, oxychlorination or hydrochlorination of non-aromatic hydrocarbons, including the reverse processes, i.e. dechlorination or dehydrochlorination. A recent report compiled from case studies of emissions from EDC/VCM processes in Europe estimates annual emissions of what are referred to as toxic equivalents of 2,3,7,8-tetrachloro dibenzodioxin (TCDD) at 1.8 kg. See De Vorming van PCDFs, PCDDs en Gerelateerde Verbindingen Bij de Oxychlorination van Etheen, Evers, E. H. G., Dept. of Environmental Toxicology, University of Amsterdam, Contract No. ZH4005, Nat. Water Service District of So. Holland (hereinafter referred to as the NWS Study) 1989. MTC Pub. No. MTC89EE. Moreover, experts are in disagreement over what levels of these toxins are generated from both natural and industrial processes.
There are numerous literature citations of investigations concerning the various synthetic routes giving rise to PCDD/Fs. Chemosphere, Vol. 23, Nos. 8-10, O. Hutzinger, Editor, Pergamon Press, 1991. By way of summary, the significant precursors that may lead to the formation of PCDD/Fs noted in the literature include chlorophenol, benzene, chlorinated benzene, and diphenyl ether. These species have been found to convert to PCDD/Fs via condensation, free-radical and ionic mechanisms. Chlorinated Dioxins and Related Compounds, O. Hutzinger, et. al, Editors, Pergamon Press, 1982.
The de novo synthesis of PCB's and PCDD/Fs from non-aromatic precursors (including elemental forms) has been reported in the literature for oxychlorination processes. Chemosphere, Eklund, G.; Pederson, J. R.; Stromberg, B., 17, 575, 1988. The NWS study demonstrated that a number of PCDD/Fs were detected in the gas phase and catalyst residue of a simulated oxychlorination process. The total amount of PCDD/F's observed in the NWS experiments was 546 nanograms per 1.31 grams of EDC formed or 417 ng/g (ppb). The PCDD/Fs congener pattern formed in the NWS experiments was very similar to that found in the waste sludge of the VCM industries along the Rhine river basin and indicates that the laboratory study accurately modeled the process occurring in commercial units. The authors of the NWS Study concluded that copper catalyst plays a role in formation of PCDD/Fs in mediums in which carbon, chlorine, oxygen and active catalytic surfaces are present. The data generated appear to confirm conclusions of other researchers that PCDD/Fs are formed de novo in the oxychlorination of alkanes and alkenes and especially with oxychlorination of ethylene to form EDC. The term de novo is defined as formation of PCDD/Fs directly from acyclic, aliphatic hydrocarbon, such as methane and ethylene, in the presence of oxygen and HCl. The researchers presumed that aromatization of simple carbon structures was occurring in order to account for the apparent de novo synthesis.
De novo formation of chlorinated alkenes, benzenes,-phenols and biphenyl has also been reported to be formed by the reaction of CO.sub.2 and HCl in the presence of catalysts. Chemosphere, Stromberg, B. 27, 179, 1993; Chemosphere, 23, 1515, 1990. These catalysts include many commercially used materials like those used in oxychlorination processes. In addition, fly ash from municipal solid waste incinerators has also been shown to catalyze formation of PCDD/Fs. a) Chemosphere, Ross, B. J.; Naikwadi, K. P.; Karasek, F. W., 19,291, 1989; b) Chemosphere, Benfenati, E.; Mariani, G.; Fanelli, R.; Zuccotti, S., 22, 1045, 1992; and Chemosphere, Born J. G. P.; Louw, R.; Mulder, P., 26, 2087, 1993. One such study has shown that favorable conditions occur at temperatures between 280.degree. C. and 300.degree. C., leading to a consensus among investigators of incineration methods that PCDD/Fs likely form in the post-combustion vent gas cooling zones of solid waste incinerators. Thus, studies have shown that de novo synthesis of chlorinated aromatic compounds apparently occurs under a variety of conditions.
With the many reports of de novo formation of PCDD/Fs in industrial chlorine consuming processes, reduction or elimination of these by-products is highly sought. Some approaches have focused on improving the reaction selectivity to making the desired product. For example, one approach which is considered from the standpoint of reduction in the level of byproducts produced in the manufacture of EDC, is the improved direct chlorination process disclosed in U.S. Pat. No. 4,410,747 ('747). This liquid phase reaction of ethylene and chlorine as taught in '747 is conducted at the boiling point of the EDC liquid in the presence of a metal chloride catalyst and added aromatic hydrocarbon, such as benzene. The side reactions forming undesired by-products, principally 1,1,2-trichloroethane, are reduced. Following the view that de novo synthesis of PCDD/F's is occurring, improved reaction selectivity could reduce the level of de novo PCDD/Fs formed.
Other improvements have focused on reduction in the volume of effluent discharged from the oxychlorination process. In the balanced process for oxychlorination of ethylene to make EDC, there is provided the recapture and reuse of hydrogen chloride from the cracking of 1,2-dichloroethane to vinyl chloride monomer. The amount of waste effluent is reduced considerably in the re-use of HCl. This recycled HCl is referred to herein as "internally sourced" and this internally sourced HCl is typically separated from EDC, VCM and byproducts by distillation.
Recent developments in the world-wide chemical industry are leading to a shift in the manufacturing of VCM away from the balanced VCM process, and towards the increasing use of HCl from external, i.e., non-oxychlorination processes. The present invention is concerned with the use of this "external" HCl, i.e., from sources other than that obtained in separating HCl from the effluent generated in the thermal cracking of the oxychlorination product, e.g., EDC. External HCl is produced in industrial processes such waste incineration of chlorinated materials; the manufacture of organic isocyanates; high temperature chlorinolysis of C.sub.1 -C.sub.3 hydrocarbons; and the dehydrochlorination of chlorinated materials such as chlorine containing polymers, chlorinated hydrocarbons, chloroacetic acid, CFC's, HCFC's and other industrially useful materials.